Contradictory hydrological impacts of afforestation in the humid tropics evidenced by long-term field monitoring and simulation modelling

Abstract

Abstract. The humid tropics are exposed to an unprecedented modernisation of agriculture involving rapid and mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability which controls habitats, water resources, and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydrometeorological variables has been operating in several headwater catchments in tropical southeast Asia since 2000. The GR2M water balance model, repeatedly calibrated over successive 1-year periods and used in simulation mode with the same year of rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses, and trend detection tests allowed causality between land-use changes and changes in seasonal streamflow to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow led to intricate streamflow patterns: pluri-annual streamflow cycles induced by the shifting system, on top of a gradual streamflow increase over years caused by the spread of the plantations.

Hydrological Regime and Water Shortage as Drivers of the Seasonal Incidence of Diarrheal Diseases in a Tropical Montane Environment

“…In the context of climate change, annual rainfall in South-East Asia is expected to increase [86]. However, combined to the expansion of higher evaporative demand perennial crops, groundwater recharge may decrease [87] and be less available for human needs. Concomitantly, more intense rainfall and land use changes towards annual crops or trees without understorey may increase runoff and decrease water travel times downstream, thereby reducing the filtration process of pathogens before entering the stream.…”

Hydrological modeling of Fecal Indicator Bacteria in a tropical mountain catchment

“…See Neitsch et al (2009) -The soil map (Chaplot et al, 2005;Valentin et al, 2008b) with associated soil properties (Shrestha et al, 2013). -The 2012 land use map from de Rouw et al (2015) and Lacombe et al (2016) with associated management practices: forest and teak plantations with mature trees at the beginning of the simulation, simplified management operations for crops. Fertilization through grazing was used to introduce E. coli bacteria into the system.…”

“…The simulated annual average of the contribution of surface runoff to stream flow was about 45% whereas the groundwater (lateral flow and shallow aquifer) contribution was 55%. The hydrological response of the Houay Pano headwater catchment is known to be very sensitive to land use (Lacombe et al, 2016), which determines the infiltration and runoff rates. The high contribution of both lateral flow and the shallow aquifer may be related to the high infiltration rates in fallow and forest land uses together with run-off/run-on processes and soil crusted areas across the catchment (Patin et al, 2012;Ribolzi et al, 2011b;Ziegler et al, 2004).…”

Impact of LUCC on streamflow based on the SWAT model over the Wei River basin on the Loess Plateau in China

“…5) showed that the observed precipitation in the study area did not decrease significantly from 1960 to 2009, while the annual streamflow (except region 3) and the runoff coefficients decreased significantly (P < 0.05) under this meteorological condition. This discrepancy could be attributed mostly to LUCC (Lacombe et al, 2008(Lacombe et al, , 2016. In order to analyze the impact of LUCC on the streamflow, the land use data for 1980 and 2005 were used in the validated SWAT model and the DEM, and the soil data remained constant.…”

“…Some researchers have indicated that tree planting has both negative and positive effects on water resources and that the overall effect was the result of a balance between them, which was strongly dependent on tree density (Tobella et al, 2014). Lacombe et al (2016) found that soil infiltrability was an important factor in explaining how two modes of afforestation (natural regeneration vs. planting) led to opposite changes in the streamflow regime. Huang (1982) analyzed Soviet research results and found that 48 % of runoff coefficients increased, 32 % had no change, and 20 % decreased with a watershed forest increase.…”